Build error in C#8

[kyranet]

Description of the issue

I'm getting a failing build in this LGTM build:

[2020-03-01 15:05:22] [build] Done Building Project "/opt/src/Skyra.sln" (rebuild target(s)) -- FAILED.
[2020-03-01 15:05:22] [build] Build FAILED.
[2020-03-01 15:05:22] [build] "/opt/src/Skyra.sln" (rebuild target) (1) ->
[2020-03-01 15:05:22] [build] "/opt/src/Skyra/Skyra.csproj" (Rebuild target) (2) ->
[2020-03-01 15:05:22] [build] (CoreCompile target) -> 
[2020-03-01 15:05:22] [build]   Monitors/CommandHandlerMonitor.cs(141,4): error CS0518: Predefined type 'System.IAsyncDisposable' is not defined or imported [/opt/src/Skyra/Skyra.csproj]
[2020-03-01 15:05:22] [build]     0 Warning(s)
[2020-03-01 15:05:22] [build]     1 Error(s)
[2020-03-01 15:05:22] [build] Time Elapsed 00:00:38.31

The error in here points to the line:

await using var db = new SkyraDatabaseContext();

The build is done from this Pull Request.

The code compiles and runs perfectly locally, but it fails in LGTM.

[jketema]

LGTM has been shutdown for quite a while at this point. Closing this issue. If this is still a problem when running CodeQL in a GitHub action, please open a new issue.